4.6: Dividing by Decimals

Introduction

The Sand Experiment

Most students love to participate in hands-on projects, and the students in Mrs. Andersen’s class aren’t any exception. At the science museum there is a whole section that is a Discovery Center. In the Discovery Center, students can use real objects to work on experiments.

Mrs. Andersen has asked her students to bring a notebook and a pencil into the Discovery Center. The students need to keep track of the experiments that they work on. They will each have an opportunity to share their discoveries when they return to the classroom.

When Miles enters the Discovery Center he is immediately overwhelmed with all of the options. After looking around, he finally decides to work on an experiment that involves an hour glass. To complete the experiment, Miles needs to figure out how long it takes 1.25 pounds of sand to go through the hour glass. There is bucket of sand that is 6.25 pounds in front of Miles. He has a scale and another bucket to hold the sand he needs for his experiment.

Miles needs to complete the experiment as many times as he can with the 6.25 pound bucket of sand. Miles picks up the scoop and begins to sort out the sand. Remember he needs 1.25 pounds of sand each time he does the experiment.

If Miles needs 1.25 pounds of sand, how many times can he complete the experiment if he has a 6.25 pound bucket?

Pretend you are Miles. If you were completing this experiment, how many times could you do it given the amount of sand you have been given and the amount of sand that you need?

In this lesson, you will find all of the information that you need. Dividing decimals by other decimals will help you with this experiment.

What You Will Learn

In this lesson you will learn to:

Divide decimals by decimals by rewriting divisors as whole numbers.

Find quotients of decimals by using additional zero placeholders.

Solve real-world problems involving division by decimals.

Time to Practice

I. Divide Decimals by Decimals by Rewriting Divisors as Whole Numbers

In our introductory problem, Miles is working on dividing up sand. If you were going to complete this problem yourself, you would need to know how to divide decimals by decimals.

How can we divide a decimal by a decimal?

To divide a decimal by a decimal, we have to rewrite the divisor. Remember that the divisor is the number that is outside of the division box. The dividend is the number that is inside the division box.

Let’s look at an example.

Example

\begin{align*}2.6 \overline{)10.4 \;}\end{align*}2.6)10.4¯¯¯¯¯¯¯¯¯¯¯¯

In this problem, 2.6 is our divisor and 10.4 is our dividend. We have a decimal being divided into a decimal. Whew! This seems pretty complicated. We can make our work simpler by rewriting the divisor as a whole number.

How can we do this?

Think back to the work we did in the last section when we multiplied by a power of ten. When we multiply a decimal by a power of ten we move the decimal point one place to the right.

We can do the same thing with our divisor. We can multiply 2.6 times 10 and make it a whole number. It will be a lot easier to divide by a whole number.

In this example, we want to make our divisor .45 into a whole number by multiplying it by a power of ten. We can multiply it by 100 to make it a whole number. Then we can do the same thing to the dividend.

Take a minute to check your rewrite and quotient with a peer. Is your work accurate?

II. Find Quotients of Decimals by Using Additional Zero Placeholders

The decimals that we divided in the last section were all evenly divisible. This means that we had whole number quotients. We didn’t have any decimal quotients.

What can we do if a decimal is not evenly divisible by another decimal?

If you think back, we worked on some of these when we divided decimals by whole numbers. When a decimal was not evenly divisible by a whole number, we had to use a zero placeholder to complete the division.

Here is a blast from the past problem.

Example

\begin{align*}5 \overline{)13.6 \;}\end{align*}5)13.6¯¯¯¯¯¯¯¯¯¯¯¯

When we divided 13.6 by 5, we ended up with a 1 at the end of the division. Then we were able to add a zero placeholder and finish finding a decimal quotient. Here is what this looked like.

Sometimes, you will need to add more than one zero. The key is to use the zero placeholders to find a quotient that is even without a remainder.

Real Life Example Completed

The Sand Experiment

Congratulations you have finished the lesson! Now you are ready for the experiment.

Here is the problem once again.

Most students love to participate in hands-on projects, and the students in Mrs. Andersen’s class aren’t any exception. At the science museum there is a whole section that is a Discovery Center. In the Discovery Center, students can use real objects to work on experiments.

Mrs. Andersen has asked her students to bring a notebook and a pencil into the Discovery Center. The students need to keep track of the experiments that they work on. They will each have an opportunity to share their discoveries when they return to the classroom.

When Miles enters the Discovery Center he is immediately overwhelmed with all of the options. After looking around, he finally decides to work on an experiment that involves an hour glass. To complete the experiment, Miles needs to figure out how long it takes 1.25 pounds of sand to go through the hour glass. There is bucket of sand that is 6.25 pounds in front of Miles. He has a scale and another bucket to put the sand he needs for his experiment.

Miles needs to complete the experiment as many times as he can with the 6.25 pound bucket of sand. Miles picks up the scoop and begins to sort out the sand. Remember he needs 1.25 pounds of sand each time he does the experiment.

If Miles needs 1.25 pounds of sand, how many times can he complete the experiment if he has a 6.25 pound bucket?

You can start by multiplying the divisor by a power of ten to rewrite it as a whole number. Do this to the dividend too. Since there are two places in the divisor, we can multiply it by 100 to make it a power of ten.

\begin{align*}125 \overline{)625 \;}\end{align*}125)625¯¯¯¯¯¯¯¯¯¯¯

Next, we divide. Our answer will tell us how many times Miles can complete the hourglass experiment.